Parameters such as bandwidth, latency, jitter, and error rate etc are collectively referred to as quality of service (QoS) parameters. Many networks provide users or applications with a mechanism to indicate their desired quality of service. For example, under the Internet Protocol, datagrams contain three bits, denominated D, T, and R, which if set request low delay (latency), high throughput, and high reliability (ie low error rate) respectively (see "Internetworking with TCP/IP; Principles, Protocols, and Architecture" by D Comer, Prentice Hall, 1988, section 7.7.4). The Internet does not guarantee to satisfy these requests, but may be influenced by them in its route selection.
In networks having many nodes, the quality of service parameters for a given communication are generally determined for the network path or route as a whole. For example, error rate and latency typically need to be summed along the relevant path. The matching of quality of service parameters across multiple nodes of a network is discussed in U.S. patent application Ser. No. 08/407,993, filed Mar. 21, 1995, and assigned to the same assignee as the present application.
Determination of quality of service across a network can be important for route selection in a network for which multiple routes exist between a given source and destination node. U.S. Pat. No. 4,905,233 discloses the selection of a route based on a link metric which is calculated from the current data rate, the link capacity (both in packets per second), and delay over the link.
Quality of service issues are particularly pertinent to networks based on asynchronous transer mode (ATM) technology, which are intended to handle a variety of forms of communications including conventional computer data transfers, and also multimedia communications. ATM technology is based on transmitting data in small packets, known as cells. Unlike most current networks, ATM networks do not provide any error detection or correction on individual cells, or any flow control. This makes ATM networks very fast, but places greater responsibility onto the applications at either end of the communication, and also on the call set-up phase. For example, the lack of flow control implies that at peak traffic rates there is the possibility of increased cell loss, if input buffers overflow.
Quality of service issues relevant to ATM and other technologies have been widely discussed in the prior art. U.S. Pat. Nos. 5,070,498, 5,335,222 and 5,432,790 describe a determination made by an ATM switch as to whether a new call of specified average and peak rate can be supported by the switch without detriment to the other calls on the switch, by determining a loading function. This approach is extended to multiple priority levels in U.S. Pat. No. 5,357,507, by calculating upper and lower call acceptance planes in a multi-dimensional space in terms of the number of calls in each of the priority levels. The use of multiple priority levels is also disclosed in GB 2285196 and U.S. Pat. No. 5,434,848. A somewhat different technique is used in U.S. Pat. No. 5,408,465, where the extra traffic that a call would cause is simulated, to determine whether or not the call can be accepted by the network. EP-A 658999 describes a call admission policy based on different classes of traffic. In "Integrated Packet Networks with Quality of Service Constraints" by W Lee and P Kamat, p223-227, IEEE Globecom '91, a mechanism is described whereby users or applications can request quality of service for a particular parameter by specifying two values, one representing the the desired or requested value, whilst the other represents the lowest acceptable value.
EP-A-621704, EP-A-632672, and EP-A-583965 illustrate the application of quality of service criteria to network technologies other than ATM, in particular to local area networks (LANS) and to fiber distributed data interface (FDDI) networks.
In many situations it is not only the link capabilities which must be verified, but also those of the sending and receiving nodes, plus intermediate nodes if appropriate. An example of this is disclosed in EP-A-494576, in which a compressed data message is to be sent between a source and target node. Prior to sending the data message, a set-up message is sent from the source node to the target node, and then back again, to confirm the level of compression supported by the source node, target node, and intermediate nodes. This message exchange represents a simple form of capability negotiation across the network.
Similar forms of capabilities negotiation are disclosed in U.S. Pat. No. 5,258,983 to set up a communication involving at least one intermediate node, and are also defined for example in ITU standard H.242, which describes a mechanism for establishing communications between audiovisual terminals using digital channels as part of the H.320 series of standards.
Whilst the above prior art illustrates that quality of service and capabilities negotiation are very important features of modern communications systems, the inventors of the present application have nevertheless recognised deficiencies that are becoming particularly significant in the emerging multimedia communications technologies.